This invention relates to weighing scales and particularly to a scale for weighing vehicles in motion.
Most conventional scales are not suitable for weighing vehicles in motion. With increased speeds, accurate measurement becomes increasingly difficult as the time available for making a measurement becomes shorter. For example, with a two-foot long platform the time available for weighing a vehicle traveling at 60 miles per hour is approximately 23 milliseconds. Factors which contribute to response time of a scale include: deflections of the load platform under loading due to lack of rigidity of the platform supporting structure, friction in the interconnections linking the load platform and base structure, and vertical travel of the load platform for activating the load cell. Improved response time for vehicle scales is difficult to achieve since improvements in rigidity necessary for rapid response, tend to result in increased friction which slows response.
Prior to the present invention, applicant evaluated a number of platform supporting mechanisms which it was believed would provide the required rigidity. These included the use of four peripheral torque transmitting bars in conjunction with the following interconnecting mechanisms: pin and links, rack and gear, loop strap, and chain and sprocket. However, it was found that all of these would have too much friction to be suitable for weighing vehicles at high speed. U.S. Pat. No. 3,354,973 issued Nov. 28, 1967 to J. C. Farquhar illustrates mechanisms similar in principle to some of those previously evaluated.
At present there is no single load cell scale capable of weighing vehicles at high speed because of lack of platform rigidity and/or speed of response. Present scales designed for weighing vehicles in motion have four or more load cells. These load cells are either of the strain gauge or the Linear Variable Differential Transformer type. Usually this means that one load cell is placed at each corner or a scale and the vehicle driving over the scale progressively loads and unloads the load cells as the vehicles move across the platform. The process of sampling these multiple load cells and reconstructing the signal for final resolution has proven difficult and costly.
Prior hydraulic load cells are not suitable for highway weighing as they have too slow response due to high amounts of vertical travel. The prior load cells have diaphragms with rolled or folded edges and the elasticity of the diaphragm results in excessive vertical travel.